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Embarrassing Questions, German Tanks and Estimations

Q: You are conducting a survey and want to ask an embarrassing yes/no question to subjects. The subjects wouldn't answer that embarrassing question honestly unless they are guaranteed complete anonymity. How would you conduct the survey?

A: One way to do this is to assign a fair coin to the subject and ask them to toss it in private. If it came out heads then answer the question truthfully else toss the coin a second time and record the result (heads = yes, tails = no). With some simple algebra you can estimate the proportion of users who have answered the question with a yes.

Assume total population surveyed is \(X\). Let \(Y\) subjects have answered with a "yes". Let \(p\) be the sort after proportion. The tree diagram below shows the user flow.

The total expected number of "yes" responses can be estimated as
$$
\frac{pX}{2} + \frac{X}{4} = Y
$$
which on simplification yields
$$
p = \big(\frac{4Y}{X} - 1\big)\frac{1}{2}
$$

Q: A bag contains unknown number of tiles numbered in serial order \(1,2,3,...,n\). You draw \(k\) tiles from the bag without replacement and find the maximum number etched on them to be \(m\). What is your estimate of the number of tiles in the bag?

A: This and problems like these are called German War Tank problems. During WW-II German tanks were numbered in sequential order when they were manufactured. Allied forces needed an estimate of how many tanks were deployed and they had a handful of captured tanks and their serial numbers painted on them. Using this, statisticians estimated the actual tanks to be far lower than what intelligence estimates had them believe. So how does it work?

Let us assume we draw a sample of size \(k\). The maximum in that sample is \(m\). If we estimate the maximum of the population to be \(m\) then probability of the sample maximum to be \(m\) is
$$
P(\text{Sample Max} = m) = \frac{m-1 \choose k-1}{N \choose k}
$$
The \(-1\) figures because the maximum is already taken out of the sample leaving behind \(m - 1\) to choose \(k -1 \) from. The expected value of the maximum using this strategy is thus
$$
E(\text{Maximum}) = \sum_{m=k}^{m=N}m\frac{m-1 \choose k-1}{N \choose k}
$$
Note, we run the above summation from \(k\) to \(N\) as for \(m < k\) the expectation is \(0\) because the sample maximum has to be at least \(k\). After a series of algebraic manipulations ( ref ) the above simplifies to
$$
E(\text{Maximum}) = M\big( 1 + \frac{1}{k}\big) - 1
$$
which is quite an ingenious and simple way to estimate population size given serial number ordering.

If you are looking to buy some books on probability theory here is a good list.

Discovering Statistics Using R
This is a good book if you are new to statistics & probability while simultaneously getting started with a programming language. The book supports R and is written in a casual humorous way making it an easy read. Great for beginners. Some of the data on the companion website could be missing.

Linear Algebra (Dover Books on Mathematics)
An excellent book to own if you are looking to get into, or want to understand linear algebra. Please keep in mind that you need to have some basic mathematical background before you can use this book.

Linear Algebra Done Right (Undergraduate Texts in Mathematics)
A great book that exposes the method of proof as it used in Linear Algebra. This book is not for the beginner though. You do need some prior knowledge of the basics at least. It would be a good add-on to an existing course you are doing in Linear Algebra.

Follow @ProbabilityPuzIf you are looking to learn time series analysis, the following are some of the best books in time series analysis.

Introductory Time Series with R (Use R!)
This is good book to get one started on time series. A nice aspect of this book is that it has examples in R and some of the data is part of standard R packages which makes good introductory material for learning the R language too. That said this is not exactly a graduate level book, and some of the data links in the book may not be valid.

Econometrics
A great book if you are in an economics stream or want to get into it. The nice thing in the book is it tries to bring out a oneness in all the methods used. Econ majors need to be up-to speed on the grounding mathematics for time series analysis to use this book. Outside of those prerequisites, this is one of the best books on econometrics and time series analysis.